#include <stdio.h>
#include <stdlib.h>
#include <math.h>
int main(void)
{
	float a,b,c,s,area;
	printf("input three numbers:");
	scanf("%f %f %f",&a,&b,&c);
	if((a + b) <= c || (a + c) <= b || (b + c) <= a)
	{
		printf("input error\n");
		exit(0);
	}
	s = (a + b + c) / 2.0;
	area = sqrt(s * (s - a) * (s - b) * (s - c));
	printf("area is %f\n",area);
	return 0;
}
